Photosynthesis
Energy and Life
• Living things need energy to survive.
• This energy comes from food. The energy in
most food comes from the sun.
Autotrophs and Heterotrophs
• Plants and some other types of organisms are able to
use light energy from the sun to produce food.
• Autotrophs
– Make their own food
– plants
• Heterotrophs
– Obtain energy from the foods they consume
– animals
Chemical Energy and ATP
• Energy comes in many forms including light, heat, and
electricity.
• Energy can be stored in chemical compounds.
• An important chemical compound that cells use to store and
release energy is adenosine triphosphate, abbreviated ATP.
• ATP is used by all types of cells as their basic energy source.
ATP
• ATP consists of:
– adenine
– ribose (a 5-carbon sugar)
– 3 phosphate groups
Phosphate groups
adenine
Sugar
ATP and Energy
• Releasing Energy
– Energy stored in ATP is released by breaking the
chemical bond between the second and third
phosphates.
Role of ATP
• The energy from ATP is needed for many
cellular activities, including active transport
across cell membranes, protein synthesis and
muscle contraction.
Check for Understanding
• With your partner, discuss the following
questions:
– What does ATP stand for?
– How is energy released from ATP?
– What is the energy released from ATP used for?
Photosynthesis
• Photosynthesis is
the process in
which green
plants use the
energy of sunlight
to convert water
and carbon
dioxide into highenergy
carbohydrates
and oxygen.
Photosynthesis
• The equation for photosynthesis is:
6CO2 + 6H2O + energy
carbon dioxide + water + sunlight
C6H12O6 + 6O2
sugars + oxygen
• Photosynthesis uses the energy of sunlight to
convert water and carbon dioxide into highenergy sugars and oxygen.
Light and Pigments
• In addition to water and carbon dioxide,
photosynthesis requires light and chlorophyll.
• Plants gather the sun's energy with lightabsorbing molecules called pigments.
– The main pigment in plants is chlorophyll.
– There are two main types of chlorophyll:
• chlorophyll a
• chlorophyll b
Light and pigments
• Light is a form of energy, so any compound
that absorbs light also absorbs energy from
that light.
• When chlorophyll absorbs light, much of the
energy is transferred directly to electrons in
the chlorophyll molecule, raising the energy
levels of these electrons.
• These high-energy electrons are what make
photosynthesis work.
Check for Understanding
• Answer the following questions with your
partner:
– What is the equation for photosynthesis?
– What is photosynthesis?
– What are pigments and how are they used in
photosynthesis?
– CRITICAL THINKING: What happens to the
pigments during the fall that causes the color of
leaves to change?
Chloroplast
• Site of photosynthesis
• Chloroplasts contain thylakoids—saclike
photosynthetic membranes.
• A stack of thylakoids is called a granum
Granum
Reactions of Photosynthesis
• Light-Dependent Reactions
– The light-dependent reactions require light.
– The light-dependent reactions produce oxygen gas
and convert ADP and NADP+ into the energy
carriers ATP and NADPH.
• NADP+ is an electron carrier that transports high
energy electrons from the light reaction to the Calvin
Cycle
Reaction of Photosynthesis
• The Calvin cycle uses ATP and NADPH from the
light-dependent reactions to produce highenergy sugars.
• Because the Calvin cycle does not require
light, these reactions are also called the lightindependent reactions.
Check for Understanding
• With your partner discuss the following
question:
– Compare and contrast the light dependent and
light independent reactions. Make a note of how
they are interconnected  how does the one
process make the other process possible?
Reaction of Photosynthesis
• The two sets of photosynthetic reactions work
together.
– The light-dependent reactions trap sunlight
energy in chemical form.
– The light-independent reactions use that chemical
energy to produce stable, high-energy sugars from
carbon dioxide and water.
Reaction of Photosynthesis